Potassium is the most abundant cation in the intracellular fluid and plays an important role in normal human physiology, especially with regard to the firing of action potential in nerve and muscle cells (Giebisch G. Am J Physiol. 1998, 274(5), F817-33). Total body potassium content is about 50 mmol/kg of body weight, which translates to approximately 3500 mmols of potassium in a 70 kg adult (Ahmed, J. and Weisberg, L. S. Seminars in Dialysis 2001, 14(5), 348-356). The bulk of total body potassium is intracellular (˜98%), with only approximately 70 mmol (˜2%) in the extracellular space (Giebisch, G. H., Kidney Int. 2002 62(5), 1498-512). This large differential between intracellular potassium (˜120-140 mmol/L) and extracellular potassium (˜4 mmol/L) largely determines the resting membrane potential of cells. As a consequence, very small absolute changes in the extracellular potassium concentration will have a major effect on this ratio and consequently on the function of excitable tissues (muscle and nerve) (Weiner, I. D. and Wingo, C. S., J Am. Soc. Nephrol. 1998, 9, 1535-1543). Extracellular potassium levels are therefore tightly regulated.
Two separate and cooperative systems participate in potassium homeostasis, one regulating external potassium balance (the body parity of potassium intake vs. potassium elimination) while the other regulates internal potassium balance (distribution between intracellular and extracellular fluid compartments) (Giebisch, Kidney Int. 2002). Intracellular/extracellular balance provides short-term management of changes in serum potassium, and is primarily driven physiologically by the action of Na+, K+-ATPase “pumps,” which use the energy of ATP hydrolysis to pump Na and K against their concentration gradients (Giebisch, Kidney Int. 2002). Almost all cells possess an Na+, K+-ATPase (Palmer, B. F., Clin. J. Am. Soc. Nephrol. 2015, 10(6), 1050-60). Body parity is managed by elimination mechanisms via the kidney and gastrointestinal tract: in healthy kidneys, 90-95% of the daily potassium load is excreted through the kidneys with the balance eliminated in the feces (Ahmed, Seminars in Dialysis 2001).
Due to the fact that intracellular/extracellular potassium ratio (Ki:Ke ratio) is the major determinant of the resting membrane potential of cells, small changes in Ke (i.e., serum [K]) have profound effects on the function of electrically active tissues, such as muscle and nerve. Potassium and sodium ions drive action potentials in nerve and muscle cells by actively crossing the cell membrane and shifting the membrane potential, which is the difference in electrical potential between the exterior and interior of the cell. In addition to active transport, K+ can also move passively between the extracellular and intracellular compartments. An overload of passive K+ transport, caused by higher levels of blood potassium, depolarizes the membrane in the absence of a stimulus. Excess serum potassium, known as hyperkalemia, can disrupt the membrane potential in cardiac cells that regulate ventricular conduction and contraction. Clinically, the effects of hyperkalemia on cardiac electrophysiology are of greatest concern because they can cause arrhythmias and death (Kovesdy, C. P., Nat. Rev. Nephrol. 2014, 10(11), 653-62). Since the bulk of body parity is maintained by renal excretion, it is therefore to be expected that as kidney function declines, the ability to manage total body potassium becomes impaired.
The balance and regulation of potassium in the blood requires an appropriate level of intake through food and the effective elimination via the kidneys and digestive tract. Under non-disease conditions, the amount of potassium intake equals the amount of elimination, and hormones such as aldosterone act in the kidneys to stimulate the removal of excess potassium (Palmer, B. F. Clin. J. Am. Soc. Nephrol. 2015, 10(6), 1050-60). The principal mechanism through which the kidneys maintain potassium homeostasis is the secretion of potassium into the distal convoluted tubule and the proximal collecting duct. In healthy humans, serum potassium levels are tightly controlled within the narrow range of 3.5 to 5.0 mEq/L (Macdonald, J. E. and Struthers, A. D. J. Am. Coll. of Cardiol. 2004, 43(2), 155-61). As glomerular filtration rate (GFR) decreases, the ability of the kidneys to maintain serum potassium levels in a physiologically normal range is increasingly jeopardized. Studies suggest that the kidneys can adjust to a decrease in the number of nephrons by increasing potassium secretion by the surviving nephrons, and remain able to maintain normokalemia. However, as kidney function continues to decline these compensatory mechanisms cannot respond to potassium load and serum K increases (Kovesdy, Nat. Rev. Nephrol. 2014). Potassium homeostasis is generally maintained in patients with advanced CKD until the glomerular filtration rate (GFR; a measure of kidney function) falls below 10-15 mL/min. At this point, compensatory increases in the secretory rate of K+ in remaining nephrons cannot keep up with potassium load (Palmer, J. Am. Soc. Nephrol. 2015). Excessive levels of potassium build up in the extracellular fluid, hence leading to hyperkalemia.
Hyperkalemia is a clinically significant electrolyte abnormality that can cause severe electrophysiological disturbances, including cardiac arrhythmias and death. Hyperkalemia is defined as a serum potassium level above the normal range, typically >5.0 mmol/L (Kovesdy, Nat. Rev. Nephrol. 2014). Moderate hyperkalemia (serum potassium above 6.0 mEq/L) has been reported to have a 1-day mortality rate up to 30 times higher than that of patients with serum potassium less than 5.5 mEq/L (Einhorn, L. M., et als. Arch Intern Med. 2009, 169(12), 1156-1162). Severe hyperkalemia (serum K+ of at least 6.5 mmol/L) is a potentially life-threatening electrolyte disorder that has been reported to occur in 1% to 10% of all hospitalized patients and constitutes a medical emergency requiring immediate treatment (An, J. N. et al., Critical Care 2012, 16, R225). Hyperkalemia is caused by deficiencies in potassium excretion, and since the kidney is the primary mechanism of potassium removal, hyperkalemia commonly affects patients with kidney diseases such as chronic kidney disease (CKD; Einhorn, Arch Intern Med. 2009) or end-stage renal disease (ESRD; Ahmed, Seminars in Dialysis 2001). However, episodes of hyperkalemia can occur in patients with normal kidney function, where it is still a life-threatening condition. For example, in hospitalized patients, hyperkalemia has been associated with increased mortality in patients both with and without CKD (Fordjour, K. N., et al Am. J. Med. Sci. 2014, 347(2), 93-100).
While CKD is the most common predisposing condition for hyperkalemia, the mechanisms driving hyperkalemia typically involve a combination of factors, such as increased dietary potassium intake, disordered distribution of potassium between intracellular and extracellular compartments and abnormalities in potassium excretion. These mechanisms can be modulated by a variety of factors with causality outside of CKD. These include the presence of other comorbidities, such as type 2 diabetes mellitus (T2DM), cardiovascular disease (CVD) or the use of co-medications that can disrupt potassium homeostasis as side effects, such as blockade of the renin-angiotensin-aldosterone system (RAAS). These contributing factors to hyperkalemia are described below.
In clinical practice, CKD is the most common predisposing condition for hyperkalemia (Kovesdy, Nat. Rev. Nephrol. 2014). Other common predisposing conditions, often comorbidities with CKD, include both type 2 diabetes mellitus (T2DM) and cardiovascular disease (CVD), both of which are linked to the development of hyperkalemia through different mechanisms. Insulin deficiency and hypertonicity caused by hyperglycemia in patients with diabetes contributes to an inability to disperse high acute potassium loads into the intracellular space. Furthermore, diabetes mellitus is associated with hyporeninemic hypoaldosteronism and the resultant inability to upregulate tubular potassium secretion (Kovesdy, Nat. Rev. Nephrol. 2014). Cardiovascular disease (CVD) and other associated conditions, such as acute myocardial ischaemia, left ventricular hypertrophy and congestive heart failure (CHF), require various medical treatments that have been linked to hyperkalaemia. For example, β2-adrenergic-receptor blockers, which have beneficial antihypertensive effects via modulation of heart rate and cardiac contractility, contribute to hyperkalemia through inhibition of cellular adrenergic receptor-dependent potassium translocation, causing a decreased ability to redistribute potassium to the intracellular space (Weir, M. A., et al., Clin. J. Am. Soc. Nephrol. 2010, 5, 1544-15515). Heparin treatment, used to manage or prevent blood clots in CVD, has also been linked to hyperkalemia through decreased production of aldosterone (Edes, T. E., et al., Arch. Intern. Med. 1985, 145, 1070-72)). Cardiac glycosides such as digoxin—used to help control atrial fibrillation and atrial flutter-inhibit cardiac Na+/K+-ATPase, but also modulate the related Na+/K+-APTases in the nephrons. This can inhibit the ability of the kidney to secrete potassium into the collecting duct and can also cause hyperkalemia.
Hyperkalemia occurs especially frequently in patients with CKD who are treated with certain classes of medications, such as angiotensin-converting-enzyme (ACE) inhibitors, angiotensin-receptor blockers (ARBs) or other inhibitors of the renin-angiotensin-aldosterone system (RAAS) (Kovesdy, Nat. Rev. Nephrol. 2014). The RAAS is important for the regulation of blood pressure, and the maximum doses of RAAS inhibitors are widely recommended for patients with hypertension, heart failure (HF), chronic kidney disease (CKD), and diabetes. Large outcome studies have shown that RAAS inhibitors can significantly decrease hospitalization, morbidity, and mortality in these patients. In patients with CKD, RAAS inhibition is beneficial for some of the common comorbidities, such as congestive heart failure (CHF). However, inhibition of the RAAS pathway also promotes potassium retention and is a major cause of hyperkalemia. Even in populations without CKD, RAAS inhibitor monotherapy (treatment with a single agent) has an incidence of hyperkalemia of <2%, but this increased to ˜5% in patients receiving dual-agent RAAS inhibitor therapy. This is further exacerbated in CKD patients, where the incidence of hyperkalemia rises to 5-10% when dual therapy is administered (Bakris, G. L., et al., Kid. Int. 2000, 58, 2084-92, Weir, Clin. J. Am. Soc. Nephrol. 2010). It is therefore often difficult or impossible to continue RAAS inhibitor therapy over extended periods of time. Hyperkalemia is perhaps the most important cause of the intolerance to RAAS inhibitors observed in patients with CKD. As a consequence, hyperkalemia has led to the suboptimal use of RAAS inhibitors in the treatment of serious diseases such as CKD and heart failure (Kovesdy, Nat. Rev. Nephrol. 2014).
Congestive heart failure patients, especially those taking RAAS inhibitors, are another large group that is at risk of developing life-threatening levels of serum potassium. The decreased heart output and corresponding low blood flow through the kidneys, coupled with inhibition of aldosterone, can lead to chronic hyperkalemia. Approximately 5.7 million individuals in the US have congestive heart failure (Roger, V. L., et al., Circulation. 2012, 125, 188-197). Most of these are taking at least one RAAS inhibitor, and studies show that many are taking a suboptimal dose, often due to hyperkalemia (Choudhry, N. K. et al, Pharmacoepidem. Dr. S. 2008, 17, 1189-1196).
In summary, hyperkalemia is a proven risk factor for adverse cardiac events, including arrhythmias and death. Hyperkalemia has multiple causalities, the most common of which is chronic or end-stage kidney disease (CKD; ESRD); however, patients with T2DM and CVD are also at risk for hyperkalemia, especially if CKD is present as a comorbidity. Treatment of these conditions with commonly prescribed agents, including RAAS inhibitors, can exacerbate hyperkalemia, which often leads to dosing limitations of these otherwise proven beneficial agents. There is therefore a clear need for a potassium control regimen to not only control serum K in the CKD/ESRD population, but also permit the administration of therapeutic doses of cardio-protective RAAS inhibitor therapy.
Dietary intervention is one possible point of control for managing potassium burden, but is difficult to manage. Furthermore, in the patient population susceptible to hyperkalemia, dietary modifications often involve an emphasis on sodium restriction, and some patients switch to salt substitutes, not realizing that these can contain potassium salts (Kovesdy, Nat. Rev. Nephrol. 2014). Finally, “heart-healthy” diets are inherently rich in potassium. Ingested potassium is also readily bioavailable, and rapidly partitions into extracellular fluid. For example, the typical daily potassium intake in healthy individuals in the United States is approximately 70 mmol/d, or ˜1 mmol/kg of body weight for a 70 kg individual (Holbrook, J. T., et al., Am. J of Clin. Nutrition. 1984, 40, 786-793). Since absorption of ingested potassium from the gut into the extracellular fluid is nearly complete, and assuming ˜17 l of extracellular fluid in a 70 kg adult, this potassium burden would essentially double serum K (70 mmol/17 L=˜4 mmol/L increase). Such an increase would be lethal in the absence of compensatory mechanisms, and the fact that ESRD patients on dialysis do not die during the interdialytic interval is a testament to the integrity of the extrarenal potassium disposal mechanisms that get upregulated in ESRD (Ahmed, Seminars in Dialysis 2001). Patients with normal renal function eliminate ˜5-10% of their daily potassium load through the gut (feces). In patients with chronic renal failure, fecal excretion can account for as much as 25% of daily potassium elimination. This adaptation is mediated by increased colonic secretion, which is 2- to 3-fold higher in dialysis patients than in normal volunteers (Sandle, G. I. and McGlone, F., Pflugers Arch 1987, 410, 173-180). This increase in fecal excretion appears due to the upregulation of the amount and location of so-called “big potassium” channels (BK channels; KCNMA1) present in the colonic epithelia cells, as well as an alteration in the regulatory signals that promote potassium secretion through these channels (Sandle, G. I. and Hunter, M. Q., J Med 2010, 103, 85-89; Sorensen, M. V. Pflugers Arch—Eur J. Physiol 2011, 462, 745-752). Additional compensation is also provided by cellular uptake of potassium (Tzamaloukas, A. H. and Avasthi, P. S., Am. J Nephrol. 1987, 7, 101-109). Despite these compensatory mechanisms, ˜15-20% of the ingested potassium accumulates in the extracellular space and must be removed by dialysis. Interdialytic increases that occur over the weekend can lead to serious cardiovascular events, including sudden death. In summary, dietary intervention is both impractical and insufficient.
Serum potassium can be lowered by two general mechanisms: the first is by shifting potassium intracellularly using agents such as insulin, albuterol or sodium bicarbonate (Fordjour, Am. J. Med. Sci. 2014). The second is by excreting it from the body using 1 of 4 routes: the stool with K binding resins such as sodium polystyrene sulfonate (Na-PSS), the urine with diuretics, the blood with hemodialysis or the peritoneal fluid with peritoneal dialysis (Fordjour, Am. J. Med. Sci. 2014). Other than Na-PSS, the medications that treat hyperkalemia, such as insulin, diuretics, beta agonists and sodium bicarbonate, simply cause hypokalemia as a side effect and are not suitable as chronic treatments. Definitive therapy necessitates the removal of potassium from the body. Studies have confirmed that reducing serum potassium levels in hyperkalemia patients actually reduces the mortality risk, further solidifying the role of excess potassium in the risk of death. One study found that treatment of hyperkalemia with common therapies both improved serum potassium levels and resulted in a statistically significant increase in survival (An, Critical Care 2012). Another study, in hospitalized patients receiving critical care, showed that the reduction of serum potassium by ≧1 mEq/L 48 hours after hospitalization also decreased the mortality risk (McMahon, G. M., et al., Intensive Care Med, 2012, 38, 1834-1842). These studies suggest that treating hyperkalemia in the acute and chronic settings can have a real impact on patient outcomes by reducing the risk of death
The potassium binder sodium polystyrene sulfonate (Na-PSS; Kayexalate) is the most common agent used in the management of hyperkalemia in hospitalized patients (Fordjour, Am. J. Med. Sci. 2014). Polystyrene sulfonate (PSS) is typically provided as a sodium salt (Na-PSS), and in the lumen of the intestine it exchanges sodium for secreted potassium. Most of this takes place in the colon, the site of most potassium secretion in the gut (and the region where K secretion appears to be upregulated in CKD). Each gram of Na-PSS can theoretically bind ˜4 mEq of cation; however, approximately 0.65 mmol of potassium is sequestered in vivo due to competing cations (e.g., hydrogen ion, sodium, calcium and magnesium). Sodium is concomitantly released. This may lead to sodium retention, which can lead to hypernatremia, edema, and possible worsening of hypertension or acute HF (Chernin, G. et al., Clin. Cardiol. 2012, 35(1), 32-36).
Na-PSS was approved in 1958 by the US FDA, as a potassium-binding resin in the colon for the management of hyperkalemia. This approval was based on a clinical trial performed in 32 hyperkalemic patients, who showed a decrease in serum potassium of 0.9 mmol/1 in the first 24 h following treatment with Na-PSS (Scherr, L. et al., NEJM 1961, 264(3), 115-119). Such acute use of Na-PSS has become common. For example, the use of potassium-binding resins has proven to be of value in the pre-dialysis CKD setting and in the management of emergency hyperkalemia, and is reportedly used in >95% of hyperkalemic episodes in the hospital setting (Fordjour, Am. J. Med. Sci. 2014). Na-PSS can be given orally or rectally. When given orally, it is commonly administered with sorbitol to promote diarrhea/prevent constipation. The onset of action is within 1-2 h and lasts approximately 4-6 hours. The recommended average daily dose is 15-60 g given singly or in divided doses (Kessler, C. et al., J. Hosp. Med. 2011, 6(3), 136-140). Kayexalate has been shown to be active in broad populations of hyperkalemic patients, including subjects both with and without chronic kidney disease (Fordjour, Am. J. Med. Sci. 2014).
There are fewer reports of the use of Na-PSS in chronic hyperkalemia, but chronic treatment is not uncommon. Chernin et al. report a retrospective study of patients on RAAS inhibition therapy that were treated chronically with Na-PSS as a secondary prevention of hyperkalemia (Chernin, Clin. Cardiol. 2012). Each patient began chronic treatment after being first treated for an acute episode of hyperkalemia (K+ levels ≧6.0 mmol/L). Fourteen patients were treated with low-dose Na-PSS (15 g once-daily) for a total of 289 months, and this regimen was found to be safe and effective. No episodes of hyperkalemia were recorded while patients were on therapy, but two subjects experienced hypokalemia which resolved when the dose of Na-PSS was reduced. Last, none of the patients developed colonic necrosis or any other life-threatening event that could be attributed to Na-PSS use (Chernin, Clin. Cardiol. 2012). Chronic treatment with once-daily Na-PSS was found safe and effective in this study.
While Na-PSS is the current standard of care treatment for potassium reduction in the U.S., the calcium salt of PSS (Ca-PSS) is also commonly used in other parts of the world, including Europe (e.g., Resonium) and Japan. All salt forms of these polymers are poorly tolerated by patients due to a number of compliance-limiting properties, including both GI side effects such as constipation, as well as dosing complexities due to dosing size and frequency, taste and/or texture which contribute to an overall low palatability. The safety and efficacy of PSS has been underexplored (by modern standards) in randomized and controlled clinical trials.
Kayexalate/Na-PSS is also poorly tolerated causing a high incidence of GI side effects including nausea, vomiting, constipation and diarrhea. In addition, Kayexalate is a milled product and consists of irregularly shaped particles ranging in size from about 1-150 μm in size, and has sand-like properties in the human mouth: on ingestion, it gives a strong sensation of foreign matter on the palate and this sensation contributes negatively to patient compliance (Schroder, C. H. Eur. J. Pediatr. 1993, 152, 263-264). In total, the physical properties and associated side-effects of Kayexalate lead to poor compliance and render the drug suboptimal for chronic use. Due to these properties, there has been a long felt need to provide an optimal drug for chronic use.
In summary, hyperkalemia is a serious medical condition that can lead to life-threatening arrhythmias and sudden death. Individuals with CKD are at particular risk; however, hyperkalemia can be a comorbidity for individuals with T2DM and CVD, and can also be exacerbated by common medications, especially RAAS inhibitors. The management of hyperkalemia involves the treatment of both acute and chronic increases in serum K+. For example, in an emergency medicine environment, patients can present with significant increases in serum K+ due to comorbidities that cause an acute impairment in the renal excretion of potassium. Examples of chronic hyperkalemia include the recurrent elevations in serum K+ that can occur during the interdialytic interval for patients with ESRD, or the persistent elevations in serum K+ that can occur in CKD patients taking dual RAAS blockade. There is thus a clear need for agents that can be used to treat hyperkalemia. Such agents, suitable for treatment of both acute and chronic hyperkalemia, while being palatable and well-tolerated by the patient, would be advantageous.